Pressure-sensitive adhesives (PSA) are materials that have tack properties at room temperature. A PSA desirably adheres to a variety of dissimilar surfaces without the need of more than finger or hand pressure. Pressure-sensitive adhesives find extensive use in the fabrication of multilayer laminate constructions, such as labels and tapes.
Conventional pressure-sensitive adhesive constructions, such as labels, generally comprise a laminate of a facestock, a pressure-sensitive adhesive layer or layers, and a coated release liner. A variety of materials may be utilized for the facestock, but typically the facestock is formed from paper or plastic films. The facestock may be a multilayer construction. The release liner provides a support or backing from which the facestock and the PSA layers are peeled away just prior to label application. The release liner often comprises a paper or other suitable substrate coated with a release layer of silicone. Pressure-sensitive tape constructions generally comprise a facestock having an adhesive coated on one side with the opposing side providing release properties, although an additional release liner may be used.
Pressure-sensitive adhesive constructions, such as tape and label constructions, are typically manufactured as a continuous roll in various widths, and are then processed or converted to form a finished product comprising commercially useful labels, label stock or tape rolls. The process of converting often involves cutting all or part of the bulk laminate roll. For example, once a pressure-sensitive construction is formed, it can be converted into finished, useable labels through a die cutting process. The die cutting process involves precision cutting through the facestock and pressure-sensitive adhesive to the surface of or partially into the release liner to form the final product's outline. A matrix stripping process can then be utilized to strip away the excess facestock and PSA between individual labels outlined during the die cutting process. Other converting operations may include butt cutting, which includes similar precision cutting, guillotining, hole punching, perforating, slitting, and printing.
In additional to material costs, the speed and width at which the converting process may proceed without breaking the facestock are critical to the cost of converting the bulk laminate PSA construction into the finished product. When the PSA construction breaks, such as a failure in the facestock, costly press shutdowns are often incurred. Thus, the faster the PSA construction can be converted without disruptive failures, the lower the cost of the finished product. While most of the current narrow-web converting presses operate at speeds of less than 152 meters per minute (500 ft/min), newer modern wide-web converting presses are designed for operation at speeds of as high as 244 meters per minute (800 ft/min) or greater, and it is desirable to manufacture PSA constructions compatible with this converting speed.
All layers of the laminate impact the converting speed, and a tremendous amount of work has been aimed at optimizing the facestock and release surfaces for faster converting. For instance, increasing matrix stripping speed generally increases stripping force, which often results in matrix breaks, which force press shutdowns. This problem may be avoided by the use of higher strength facestocks, which may convert better than low strength facestocks at a range of converting speeds.
Historically, however, the pressure-sensitive adhesive layer has presented the greatest limiting factor with respect to the speed of converting bulk laminates into finished product. The converting industry desires an adhesive layer having good flow properties that can adhere to a wide variety of substrates. It is also desirable for the PSA to exhibit excellent tack properties over a wide range of temperatures, to be suitable for single layer coating and to be easily compounded. However, known adhesive compositions that are formulated to have these desired properties typically are complicated formulations that are not easily or inexpensively compounded, may require multiple layers of differing compositions or may not always convert well, oftentimes sticking to the cutting dies, smearing on the matrix and label edges, interfering with the precision cutting, or otherwise slowing down the converting process.
To date, achieving good convertibility has not automatically coincided with achieving excellent adhesive performance, cost of adhesive and/or ease of compounding. Conventional adhesives must be formulated to fit predetermined needs. Important properties include peel adhesion, tack, shear properties, viscosity at various temperatures, and the like. Previous attempts to improve pressure-sensitive adhesives for the manufacturing and converting industry have been met with limited success. For instance, U.S. Pat. No. 6,416,858 discloses multilayer pressure-sensitive adhesive constructions purported to exhibit improved convertibility and aging. However, this patent's constructions require two adhesive layers of different adhesive composition to obtain the stated benefits. Coating two layers of adhesive is undesirable due to the additional processing steps and/or machinery required, as well to the requirement of multiple adhesive inventories to be maintained.
International Patent Application WO97/30844 discloses a pressure-sensitive adhesive including an LVSI, low molecular weight styrene-isoprene diblock copolymer that is both expensive and difficult to use due to its high viscosity at ambient temperatures and tendency to oxidize when heated unless blanketed with an inert atmosphere such as nitrogen.
U.S. Pat. No. 5,290,842 shows a combination wherein a styrene-butadiene block copolymer is combined with a styrene-isoprene-styrene block copolymer to show two separate and distinct glass transition temperature peaks, indicating less than efficient tackification of the elastomer system and tack properties over a narrower range of temperatures. Only aliphatic tackifying components are disclosed to achieve these properties. U.S. Pat. Nos. 5,856,387 and 5,663,228 are similarly directed to PSAs having two distinct glass transition temperatures and incorporate aliphatic tackifiers.
U.S. Pat. Nos. 6,384,138 and 6,162,859 are directed to adhesives where the tackifier component comprises a blend of at least two or three hydrocarbon resins, thus requiring a complex and costly formulation.
Thus, it is desirable to provide a pressure-sensitive adhesive that is cost effective, easily manufactured or compounded, that provides excellent tack properties over a wide range of temperatures, that provides good adhesion to a wide variety of substrates of varying roughness, that may be applied over a wide range of temperatures and that provides excellent convertibility properties to constructions featuring the pressure-sensitive adhesive.